The present invention is directed to a method and apparatus for determining the angular relationship between a vertically oriented, gyroscopically stabilized mass and its supporting movable body and for providing a magnetic heading of the supporting movable body as it traverses a given path on the earth. Conventional vertical gyroscopes are devices which have a gyroscopically stabilized mass which aligns itself with the earth's vertical so that departures of the supporting body relative to the gyroscopically stabilized mass are sensed for display and various control purposes. Such vertical gyroscopes have a two-gimbal system wherein mechanical angular motion of the gyroscopically stabilized mass presents itself in angular rotation of one or both of the gimbals relative to the supporting body upon which the gyroscope is affixed. Conventionally, this gimbal angular rotation is electrically measured by a sensor of some type, such as synchros, potentiometers, E pick offs, etc. and in these types of systems, the physical location required for the sensors introduces several undesirable features such as additional weight to the gimbals, additional gimbal rotation or friction due to wiring required to the sensors through slip rings and, at the most critical locations, space at the gimbal pivot points. Many attempts have been made to arrive at a solution to these problems by way of inductive couples, optical couples and the like, however, these devices represent relatively complex and expensive solutions to the pick off problem and at the same time, do not provide magnetic heading information which is usually provided by a separate gyroscope and/or flux gate compass.
Attempts have been made in the past to combine an earth inductor compass with a gyro vertical or gyroscopic artificial horizon but for various reasons these have not found acceptance in the art and conventionally, the magnetic heading information and the attitude information have been developed by separate gyroscopic instrumentalities and flux gate compasses. An example of a prior art effort to combine an earth inductor compass with a gyro vertical or gyroscopic artificial horizon is illustrated in Bentley U.S. Pat No. 2,176,197 wherein the earth inductor coil is used for driving a stroboscopic element for displaying heading and the roll and pitch attitude signals are displayed by means of mechanical movement of an artificial horizon. Utilization of a rotating field leakage of an electric gyro rotor in conjunction with a stationary coil element for sensing attitude changes by virtue of relative changes in angular orientations of the gyroscope rotor is illustrated in Wendt U.S. Pat. No. 2,737,054. In the Wendt patent, a pair of substantially annular coils mounted in a plane parallel with the main gimbal and major gyro axis of a three axis directional gyroscope has the leakage flux of the stator windings coupled to the coil in phase relationships which are directly related to the angular orientation of the coils with respect to the spin axis. Magnetic heading was not available from this device. It is also known to use a notched cylinder as a shorted coil for rotation at high speed in the earth's magnetic field and coupling to currents flowing in the shorted coil by transformer action to detect magnetic directions. Such a system is shown in Sydnes U.S. Pat. No. 2,049,232 who discloses the use of a pair of orthogonal coils for developing signals corresponding to reference position of the rotor so that the direction of the earth's magnetic field can be determined by phase comparisons in a phase comparator, with the rotor of that device being driven by an electrical motor or by high pressure air turbine with the shaft and associated coils all kept in a vertical stationary position relative to the supporting body by means of a gimbal suspension system. In Lewis U.S. Pat. No. 4,013,946, there is disclosed an earth inductor system in which the rotor is a shorted electrical path and that device utilized a shaft angle encoder in one embodiment for providing a signal corresponding to the rotor reference position and, in a second embodiment, an excitation coil, fixed with respect to the axis of the rotor, is used for generating a tertiary magnetic field at a frequency different from the field induced in the shorted winding by rotation of the rotor in the earth's magnetic field and this tertiary magnetic field induces a current flow in the shorted winding which produces a fourth magnetic field which is detected to produce an electrical signal which is utilized as a reference with respect to the signal produced by the rotation of the shorted winding in the earth's magnetic field. No attitude information was available from this device.
In accordance with the present invention, a gyroscopic device, preferably a vertical gyro, is provided with a short circuited rotor winding which is so shaped and oriented with respect to the spin axis as to produce an axial flux component and a radial flux component when the vertically maintained rotor is rotated in the earth's magnetic field. A fixed coil coaxial with the rotor and gyroscopically stabilized thereby acts as the secondary of a transformer with the electromotive force induced therein being due to the axial flux component of the rotor. In addition, at least one further coil affixed to the supporting body with respect to the gyro rotor is linked to the radial flux component and is relatively angularly movable in relation to the rotor so that a first electrical signal voltage component is induced in the fixed coil which has a phase dependent upon the angular position of a gyroscopic device with respect to the earth's magnetic field, a second electrical signal voltage component is induced in at least a pair of the angularly related coils which have a phase independent of the angular position of the gyroscope with respect to the earth's magnetic field but are quadrature voltages of double the frequency of the first electrical signal and have amplitudes dependent upon the angular relationship between the axis of the rotor and the axis of the second coils and a frequency which is harmonically related to the rotor frequency and a phase which is independent of the direction of the earth's magnetic field. This system thereby provides both magnetic heading information and attitude information in a single vertical gyro. By providing a further set of coils on the supporting body which are coaxially aligned initially with the gyroscopically stabilized coil, the system is capable of measuring the magnitude and direction of angles through 360 degrees without ambiguity in a gimballed vertical gyro system. The coils which are stationary with respect to the supporting body and orthogonally related with respect to each other are used to excite the sensing coil which is gyroscopically stabilized by the rotor. In such case, each set of excitation coils is supplied at an alternating current frequency which is sufficiently different from the other coil's excitation frequencies that frequency discrimination of the various electrical signal components of the sensor coil output voltage is possible.